Led switching arrangement

ABSTRACT

The invention relates to a switching arrangement for operating at least one LED, which switching arrangement is provided with: input terminals ( 1, 2 ) for connecting a supply source; output terminals ( 3, 4 ) for connecting the LED to be operated; a first series circuit (I) between one of the input terminals ( 1 ) and one of the output terminals ( 3 ), including at least a self-inductance (L), a capacitor (C) and a diode (D); a second series circuit (II) between the input terminals, including at least the self-inductance (L) and a switiching element (S) which is alternately switched to a conducting state and a non-conducting state at a high frequency; a third series circuit (III) between the output terminals, including the diode and an inductive winding (SW). According to the invention, the inductive winding forms a secondary winding of a transformer (T), which has a primary winding (PW) which forms part of both the first and the second series circuit.

[0001] The invention relates to a switching arrangement for operating aload, which switching arrangement is provided with

[0002] input terminals for connecting a supply source,

[0003] output terminals for connecting the load to be operated, a firstseries circuit between one of the input terminals and one of the outputterminals, including at least a self-inductance, a capacitor and adiode,

[0004] a second series circuit between the input terminals, including atleast said self-inductance and a switching element which is alternatelyswitched to a conducting state and a non-conducting state at a highfrequency,

[0005] a third series circuit between the output terminals, including atleast said diode and an inductive winding.

[0006] A switching arrangement of the kind referred to in theintroduction is known from U.S. Pat. No. 5,682,306. In the knownswitching arrangement, which is also known by the name of SEPIC (SingleEnded Primary Inductance Converter), the self-inductance forms a firstenergy storage element, and there will be a voltage across the capacitorsubstantially of the magnitude of an input voltage applied to the inputterminals. This type of converter appears to be suitable for driving anLED array comprising at least one LED as the load. LED arrays are verysuitable for use as a light source, for example in a traffic lightsinstallation, inter alia on account of their low energy consumption incomparison with incandescent lamps which are suitable for use in suchtraffic lights installations, and on account of the fact that they havea much longer life than the incandescent lamps in question. In such anapplication, the switching arrangement and the LED array in question aregenerally supplied with power from a public mains as the power supplysource.

[0007] One drawback of the known switching arrangement is the occurrenceof a relatively high level of radio interference (EMI).

[0008] It is the object of the invention to provide a measure forreducing the EMI level.

[0009] In order to achieve that objective, a switching arrangement ofthe kind referred to in the introduction as the switching arrangementaccording to the invention is characterized in that the inductivewinding forms a secondary winding of a transformer which has a primarywinding which forms part of both the first and the second seriescircuit.

[0010] In this way it is achieved that a high-frequency voltage signalacross the self-inductance caused by the high-frequency switching of theswitching element is effectively compensated in large measure. As aresult, the generation of EMI will be significantly reduced. Since fullcompensation by means of the primary winding does not occur in practice,a further reduction can be advantageously achieved by arranging a bypasscapacitor between the input terminals, which capacitor functions as abypass for the high-frequency ripple current signal generated in theself-inductance by the switching element.

[0011] An optimum result can be achieved if the arrangement isconfigured in accordance with the relation

2π[(L 1+Ls)C3]^(½)>δ,

[0012] wherein:

[0013] L1 is the magnitude of the self-inductance in H,

[0014] Ls is the magnitude of the self-inductance of the secondarywinding in H,

[0015] C3 is the capacitance of the capacitor in F, and

[0016] δ is the fraction in s of each switching period of the switchingelement during which the switching element is switched in itsnon-conducting state.

[0017] In another embodiment of the switching arrangement according tothe invention, the secondary winding is made up of a first winding and asecond winding, which second winding is incorporated in the first seriescircuit and which also has a connection point with the first winding.Thus, an auto transformer function has been effected, as a consequenceof which the periodic switching of the switching element to anon-conducting state takes place at a reduced current value. Anotherconsequence is the fact that this leads to an increase of δ. Bothconsequences have an advantageous influence as regards the reduction ofthe objectionable influence of EMI, on the one hand as a result of thereduction of the amount of EMI and on the other hand as a result of ashift to lower frequencies.

[0018] The above and further aspects of the invention will be explainedin more detail hereinafter with reference to a drawing of the switchingarrangement according to the invention. In the drawing

[0019]FIG. 1 shows a diagram of a switching arrangement according to theinvention,

[0020]FIG. 2 shows a variant of the switching arrangement as shown inFIG. 1,

[0021]FIG. 3A shows a current and voltage diagram of a switchingarrangement according to the invention, and

[0022]FIG. 3B shows a current and voltage diagram of a switchingarrangement not according to the invention.

[0023] A switching arrangement according to the invention for operatingat least one LED is shown in FIG. 1, which switching arrangement isprovided with

[0024] input terminals 1, 2 for connecting a supply source,

[0025] output terminals 3, 4 for connecting the LED to be operated,

[0026] a first series circuit I between one of the input terminals 1 andone of the output terminals 3, including at least a self-inductance L, acapacitor C and a diode D,

[0027] a second series circuit II between the input terminals 1, 2,including at least said self-inductance L and a switching element Swhich is alternately switched to a conducting state and a non-conductingstate at a high frequency,

[0028] a third series circuit III between the output terminals 3, 4,including at least the diode D and an inductive winding SW. Furthermore,a buffer capacitor CB is arranged between the output terminals 3, 4.

[0029] The inductive winding forms a secondary winding SW of atransformer T which has a primary winding PW which forms part of boththe first and the second series circuit. A connection point between theswitching element S and primary winding PW forms a drain d of theswitching element S.

[0030] In an advantageous embodiment, a bypass capacitor BYC is arrangedbetween the input terminals 1, 2.

[0031] In a variant of the switching arrangement according to theinvention which is shown in FIG. 2, the secondary winding is made up ofa first winding SW1 and a second winding SW2, which second winding isincorporated in the first series circuit I and which also has aconnection point VB with the first winding SW1. Preferably, the secondwinding SW2 is directly connected to one of the output terminals bymeans of a snubber circuit 5. The snubber circuit 5 in particularprovides a reduction of voltage peaks that occur when the switchingelement S is switched off. This helps to achieve a further reduction ofthe generation of interference signals which, surprisingly, has anegligible effect on the power transfer of the inductive winding.

[0032] A practical embodiment of a switching arrangement according tothe invention as described in FIG. 2, is in particular suitable forbeing operated on a 110V, 60 Hz supply source. The switching arrangementis suitable for operating an LED array, for example forming part of atraffic lights installation. An array which is used in practice is, forexample, of the type GR 690053224, made by Lumileds Lighting, whichcomprises eighteen LEDs, which LEDs emit green light. The switchingarrangement is capable of supplying the array with a controlled currentranging between 300 mA and 1.1 A during operation, in dependence on,inter alia, the temperature of the array.

[0033] In the switching arrangement, the self-inductance has a value L1in the order of 3900 μH, the capacitor C has a capacitance C3 of 47 nFand the secondary winding SW of the transformer T has a self-inductanceLs of 120 μH. The winding ratio of the primary winding PW and thesecondary winding SW is 1:1. The primary and the secondary windingcomprise 62 and 56 turns, respectively. The first winding SW1 and thesecond winding SW2 of the secondary winding each have 28 turns and aself-inductance of 50 μH. The buffer capacitor CB has a value of 330 μH.The switching element S is made up of a MOSFET type IRF730, made byInternational Rectifier. The value of the bypass capacitor BYC is 100nF.

[0034] When a connected array as described above is operated at arectified sine-wave voltage of 110 V, the switching element isalternately switched to a conducting and a non-conducting state duringzero-axis crossing of the supply source at a high frequency varyingbetween 50 kHz and maximally 160 kHz.

[0035]FIG. 3A shows a current and voltage diagram of the above-describedpractical switching arrangement, in which the horizontal axis forms thetime axis. The switching arrangement is fed with 120 V dc. Curve 100shows the trend of the voltage at the location of the drain d of theMOSFET that forms the switching element S. The current that flows at thelocation of the drain d is shown in graph 101.

[0036] By way of comparison, FIG. 3B shows a current and voltage diagramof a switching arrangement which comprises only one inductive winding,which is built up identically to the one that is shown in FIG. 3A. Inthe diagram, the curves 200 and 201 show, respectively, the trend of thevoltage at the location of the drain and the trend of the current at thelocation of the drain.

[0037] The fraction δ of each switching period of the switching elementduring which the switching element is switched in a non-conducting stateamounts to 7 μs. A comparison between the curves is 101 and 201 showsthat the variation in the current value has been reduced from 87 mA inthe case of curve 201 to 17 mA in the case of the switching arrangementaccording to the invention. Furthermore it is apparent that ahigh-frequency ripple occurring during the periodic switching of theswitching element to a non-conducting state has considerably decreasedin magnitude in the case of the switching arrangement according to theinvention. In this manner, the occurrence of a high-frequency signalacross the self-inductance is substantially entirely prevented as aresult of the high-frequency switching of the switching element, as aresult of which the generation of EMI has been reduced to a significantextent.

1. A switching arrangement for operating at least one LED, whichswitching arrangement is provided with input terminals for connecting asupply source, output terminals for connecting the LED to be operated, afirst series circuit between one of the input terminals and one of theoutput terminals including at least a self-inductance, a capacitor and adiode, a second series circuit between the input terminals, including atleast said self-inductance and a switching element which is alternatelyswitched to a conducting state and a non-conducting state at a highfrequency, a third series circuit between the output terminals,including at least said diode and an inductive winding, characterized inthat the inductive winding forms a secondary winding of a transformerwhich has a primary winding which forms part of both the first and thesecond series circuit.
 2. A switching arrangement as claimed in claim 1,characterized in that a bypass capacitor is arranged between the inputterminals.
 3. A switching arrangement as claimed in claim 1 or 2,characterized in that the arrangement is configured in accordance withthe relation 2π[(L 1+Ls)C 3]^(½)>δ, wherein: L1 is the magnitude of theself-inductance in H, Ls is the magnitude of the self-inductance of thesecondary winding in H, C3 is the capacitance of the capacitor in F, andδ is the fraction in s of each switching period of the switching elementduring which the switching element is switched in its non-conductingstate.
 4. A switching arrangement as claimed in claim 1 or 2,characterized in that the secondary winding is made up of a firstwinding and a second winding, which second winding is incorporated insaid first series circuit and which also has a connection point with thefirst winding.